The
biosorption of lead(II) and chromium(VI) on groundnut hull was
investigated. Batch biosorption experiments were conducted to
find the equilibrium time and biosorption capacity. Effect of
parameters like pH, temperature and initial metal concentration
was studied. The maximum biosorption capacity of lead(II) and
chromium(VI) was found to be 31.54 ± 0.63 and 30.21 ± 0.74 mg g-1,
respectively. The optimum pH for lead(II) and chromium(VI) removal was 5 ± 0.1
and 2 ± 0.1, respectively. The temperature change, in the range of 20
- 45ºC affected the biosorption capacity. The maximum removal of lead(II)
was achieved at 20 ± 2ºC, where as maximum uptake of chromium(VI)
was observed at 40 ± 2ºC. The biosorption data was fitted to the
Langmuir and the Freundlich
isotherm models. The Langmuir
model showed better representation of data, with correlation coefficient
greater than 0.98. The kinetics of biosorption followed the pseudo
second order kinetics model. The thermodynamics parameters were evaluated from
the experimental data.

Lead and chromium are the major toxic
pollutants, which entered the water streams through various industrial
operations. The potential sources of chromium(VI) wastes are effluents from
metallurgy, electroplating, leather tanning, textile dyeing, paint, ink, and aluminium
manufacturing industries (Bhattacharyya and Gupta, 2006; Verma et al. 2006). Lead
is used as industrial raw material in the manufacture of storage batteries,
pigments, leaded glass, fuels, photographic materials, solder and steel
products (Nadeem et al. 2006). Lead and chromium are toxic contaminants, even
in very low concentrations. Severe lead poisoning can cause encephalopathy,
with permanent damage, while moderate lead poisoning result in neurobehavioral
and intelligent deficit (Chen et al. 2007). The presence of lead in drinking
water, even in low concentrations may cause anaemia, hepatitis and nephritic
syndrome (Zulkali et al. 2006). Lead poisoning in humans causes severe damage
to kidney, nervous system, reproductive system, liver and brain (Ozer, 2007). Chromium
(VI) is carcinogenic to both humans and animals (Mungasavalli et al. 2007). Strong
exposure of chromium (VI) causes cancer in the digestive tract and lungs and
may cause gastric pain, nausea, vomiting, severe diarrhea, and hemorrhage (Mohanty
et al. 2005). According to the United States Environmental Protection Agency (USEPA) the maximum permissible limits in wastewater and
potable water are 0.1 mg l-1 and 0.015 mg l-1 for lead(II) and 1.0 mg l-1 and 0.05 mg l-1 for chromium(VI), respectively (Park et al. 2005).

Groundnut
hull is an agricultural based waste material and these materials
as discussed previously have the potential to sequester metals
from solutions. The activated carbon prepared from groundnut shell
has been utilized for the sorption of dyes methylene blue (Kannan
and Sundaram, 2001) and
malachite green (Malik et al. 2007). No report
on the utilization of groundnut hull, for biosorption of lead(II)
and chromium(VI) has been found in literature. The present research
was conducted to utilize groundnut hull for the biosorption of lead(II)
and chromium(VI) from wastewater. Influence of operating conditions
like temperature, pH, biosorbent dose and initial metal concentrations
on biosorption process were investigated. The equilibrium, kinetics
and thermodynamics of biosorption process were studied.

Materials and Methods

Chemicals and instruments

Experiments were performed in triplicate and average
values were used in the results. Chemicals used were of analytical reagent
grade. The stock lead(II) and chromium(VI) solutions of 1000 mg l-1concentrations
were prepared by dissolving, 1.6 g lead nitrate (Merck Germany) and 2.827 g potassium
dichromate (Fischer Germany), respectively in one litre of distilled water. The
stock solutions were further diluted to obtain, solutions of various known
concentrations of lead(II) and chromium(VI). A variable speed shaker 20-500 rpm was used for batch
experimentation. It has the capacity of holding eight Erlenmeyer flasks,
simultaneously. It was equipped with thermostatically controlled, heating water
bath. Initial concentrations of lead(II) and chromium(VI) solutions, used in
the experiments were 100 mg l-1. The rotational speed of stirrer, in
all the experiments was kept constant at 200 rpm. Except for the experiments to
study the effect of dose, 0.5 g biosorbent was used in 100 ml of solution.
After completion of each batch, the solution was filtered. The filtrate was
analyzed, using Shimadzu 6800 Atomic Absorption Spectrophotometer, to determine
the quantity of residual metals. The quantity of sorbed metal was found by
material balance. The metal uptake, qt was determined using the following
equation:

qt = V (Co - Cf)/m [1]

Where, Co and Cf are the
initial and final concentration of metal in solution (mg l-1), V is
the volume of solution (l) and m is the mass of biosorbent (g).

Preparation of biosorbent

The
groundnut hull was collected locally and washed with tap water
to remove, the attached dust and other impurities. The washed groundnut
hull was ground in a mechanical grinder to form a powder. The powder
was sieved and a size fraction in the range of 200-300 µm
was used in all the experiments. This powder was soaked (20 g/l)
in 0.1 M nitric acid for 24 hrs. The mixture was filtered and the
powder residue was washed with distilled water, several times to
remove any acid contents. This filtered biomass was first dried,
at room temperature and then in an oven at 105ºC for 6-8 hrs.
The dried biomass was stored in air tight glass bottles to protect
it from moisture.

Characterization of biosorbent

The
Fourier Transform Infrared (FTIR) spectroscopy was used to identify
the functional groups present in the biomass. The biomass samples
were examined, using JASCO FTIR spectrometer, within range of 400-4000
cm-1. All analysis were performed using, KBr as back
ground material. In order to form pellets, 0.002 g of groundnut
hull was mixed with 0.3 g KBr and pressed at 6-8 bar pressure.
The surface structure and particle size distribution of biosorbent
was examined using Hitachi Scanning Electron Microscope (SEM).
The samples were covered, with a thin layer of gold and an electron
acceleration voltage of 20 kV was applied. The surface area, pore
volume and pore size measurements of biosorbent was carried out
using, Quantachrome NOVA 2200C USA, surface area and pore size
analyzer. The gas mixture of 22.9 mole % nitrogen and 77.1 mole
% helium was used for this purpose. The elemental analysis of the
biosorbent was performed using,
Costech Instrument 4010 elemental analyzer, Thermo Jarrel Ash (IRIS,
USA) inductively couple plasma atomic emission spectrometer (ICP-AES)
and Shimazdu atomic absorption spectrometer.

Results and Discussion

Characterization
of biosorbent

The
FTIR spectra of groundnut hull is shown in Figure
1. Broad peak at 3400 cm-1 is the indicator of -OH
and -NH groups. The stretching of the -OH groups bound to methyl
radicals presented a signal between 2950 and 2887 cm-1.
The peaks located at 1737 and 1633 cm-1 are
characteristics of carbonyl group stretching from aldehydes and ketones.
The presence of -OH group, along with carbonyl group, confirms the
presence of carboxylic acid groups in the biosorbent. The peaks at
1508 cm-1 are associated with the stretching in aromatic
rings. The peaks observed at 1071 and 1024 cm-1 are due
to C-H and C-O bonds. The -OH, -NH, carbonyl and carboxylic groups
are important sorption sites (Volesky, 2003). The
FTIR spectra of the groundnut hull, attained after biosorption of
lead(II) and chromium(VI) is shown in Figure 2 and Figure
3. As compared to
simple groundnut hull, the broadening of -OH peak at 3400 cm-1 and
carbonyl group peak at 1633 cm-1 was observed. This indicates
the involvement of hydroxyl and carbonyl groups in the biosorption
of lead(II) and chromium(VI).

The
SEM micrographs of groundnut hull are shown in Figure
4. These micrographs represent a porous structure, with large
surface area. The results of surface area analysis further confirmed
the porous nature and high surface area of the biosorbent. The surface
area, pore volume, pore dia. and elemental analysis of the biosorbent
are presented in Table 1. The
elemental analysis of the biosorbent indicates the presence of Ca,
Mg, Na and K. These light metals can be exchanged with heavy metals
in an ion exchange reaction. The involvement of these metals in biosorption
was confirmed by analyzing the final solution for light metals.

Determination of equilibrium time

The
variation in metal uptake, with time is shown in Figure
5. As can be seen from this Figure, the biosorption process took
place in two stages. The first stage was rapid, where about 75% biosorption
was completed, within first 15 min. The second stage represented
a slower progressive biosorption. The rapid initial biosorption may
be attributed to the accumulation of metals on to the surface of
biosorbent, due to its large surface area. With the progressive occupation
of these sites, process became slower in the second stage. Moreover
the initially deposited metal ions penetrate to the interior of the
biosorbent through intra-particle diffusion which was slower process.
This is in accordance with the observations of other similar studies
(Sangi et al. 2008; Qaiser et al.
2009). The biosorption process attained
equilibrium in 1 hr. Based on the results of kinetics experiments,
a time of 1 hr was considered to be adequate for remaining experimentations.
The release of calcium, magnesium, sodium and potassium ions was
also observed as result of lead(II) binding to groundnut hull. The
elemental analysis shown in Table 1 indicated
the presence of these light metals in the groundnut hull. The release
of light metals as result of lead(II) biosorption to groundnut hull
is shown in Figure 6. The release of these
light metal ions revealed that ion exchange was taking place between
the lead(II) and light metals. Moreover decrease in solution pH,
in case of lead(II) biosorption was observed. This may be due to
the release of H+ ions, as result of lead(II) attachment
to the biosorbent. On the other hand, an increase in solution pH
was observed as result of chromium(VI) biosorption. This was probably
due to the release of OH- ions, in exchange with HCrO4- ions.
These results clearly indicate that ion exchange was the major removal
mechanism in the metal biosorption.

Effect of biosorbent dose

The removal efficiency and specific uptake of metals depend
on type and quantity of the biosorbent. If no information is available, for
particular type of biomass it is better to find the optimal dose,
experimentally. The quantity of biosorbent was varied from 1-50 g l-1.
As revealed in Figure 7, the percentage removal increased with increase
in biosorbent dose. There was negligible increase in the percentage removal,
beyond the dose of 20 g l-1. Considering these results, a dose of 5
g l-1 was considered sufficient, for the optimal removal of both metals.
Moreover, the biosorption capacity was high at low dose rates. The reason for this
may be the availability of lesser binding sites and these were fully utilized.

Effect of pH

In
heavy metal biosorption, pH is the most important parameter. The
speciation of metals in the solution is pH dependent. At the same
time, the state of chemically active sites is changed by the solution
pH. The solution pH was varied in the range from 1-6, by using
0.1 M nitric acid and 0.1 M ammonia as buffers. As shown in Figure
8, maximum removal
of chromium(VI) was achieved at pH 2 ± 0.1, where as lead(II)
has an optimal pH of 5 ± 0.1. At pH higher than 6 for both
metals precipitation occurred, due to this reason biosorption was
not studied beyond pH of 6. The FTIR spectra of groundnut hull indicated
the presence of hydroxyl, amino, carboxylic and carbonyl groups.
These groups are positively charged, when protonated at low pH and
are negatively charged at higher pH. So at low pH, the attachment
of lead(II) to sorption sites of groundnut hull was restricted.
Thus removal of lead(II) was repressed at low pH. With the increase
in pH, there was an increase in ligands with negative charges, which
resulted in increased binding of lead(II). However, at pH 6, the
lead uptake decreased due to its partial precipitation. At low pH,
dominant form of chromium(VI) is HCrO-4 (Cimino
et al. 2000) which was attracted by positively charged sites.
The further justification, for higher removal of chromium at low
pH is the reduction of chromium(VI) to chromium(III) (Park
et al. 2005; Qaiser et al. 2007), which was
then biosorbed by the sorbent. As a result of this reduction reaction,
H+ was consumed and an increase in pH was observed at
the end of experiments. The reduction of Cr(VI) to Cr(III) was further
confirmed by analyzing the solution for Cr(VI) using 1,5 diphenylcarbazide
in spectrophotometer.

Biosorption equilibrium

The biosorption equilibrium is established, when the
concentration of sorbate in bulk solution is in dynamic balance with that on
the liquid-sorbent interface. The degree of the sorbent affinity for the
sorbate determines its distribution between the solid and liquid phases. Several
models are often employed to interpret the equilibrium data. In the present
research, the Langmuir and the Freundlich models were utilized to explain the
experimental data.

The Langmuir model (Langmuir, 1918) is based on the
hypothesis that uptake occurs on a homogenous surface by monolayer sorption
without interaction between adsorbed molecules. It is expressed as:

qe = qmax b
Ce / 1 + b Ce [2]

Equation (2) can be written in linear form
as:

Ce / qe =
Ce / qmax + 1 / qmaxb [3]

Where, qmax represents the
maximum biosorption capacity and b is an affinity parameter, related to the energy
of biosorption.

The Freundlich model (Freundlich, 1928) proposes
a monolayer sorption with heterogeneous energetic distribution of active sites
and with interaction between adsorbed molecules. It is expressed mathematically
as:

qe = KF (Ce)1/n [4]

Equation [4] can be written in linear form
as:

ln qe = 1 / n ln Ce + ln KF [5]

Where; KF and n are the Freundlich
coefficients. The constant KF provides an indication of the biosorption capacity of
biosorbent and n is related to the intensity of biosorption.

Initial concentration of both metals was varied from
10 to 1000 mg l-1and quantity of biosorbent
was kept constant at 0.5 g. Equilibrium concentration, Ce and
equilibrium capacity, qe were determined. Ce was plotted
against Ce / qe and straight lines as revealed in Figure
9 were fitted by regression.

In order to see the applicability of Freundlich model,
ln Ce was plotted against ln qe and straight lines as
shown in Figure 10 were fitted to the plots. The values of the Langmuir
and the Freundlich model parameters were evaluated from the slope and intercept
of lines in Figure 9 and Figure 10 and are presented in Table
2. The experimentally determined equilibrium isotherms, for both metals
were compared with the theoretical Langmuir and Freundlich isotherms. This
comparison is shown in Figure 11, which depicts that the Langmuir model gave
better representation of the experimental data.

Biosorption kinetics

In order to investigate the biosorption
kinetics, the Lagergren first order (Lagergren, 1898) and pseudo second order
kinetics models (Ho and Mckay, 1999) were applied. The pseudo second order
model represented the data more appropriately, with correlation coefficient
greater than 0.99.

The expression for the Lagergren first order
model is:

dqt / dt = k1 (qe - qt ) [6]

This equation [6] can be integrated to yield a linearized
form as

log (qe - qt )= log qe - k1 t / 2.303 [7]

Where, k1 is the Lagergren rate
constant for adsorption (min.-1), qe is the amount of
metal biosorbed at equilibrium (mg g-1) and qt is the
amount of metal biosorbed (mg g-1) at any time t.

Time, t was plotted against log (qe - qt ) and straight lines, shown in Figure 12 were fitted by
regression. The values of k1 and qe were determined from the
slope and intercept of lines in Figure 12 and are presented in Table
2.

The equation of pseudo second order model
is:

dqt /
dt = k2 (qe - qt)2 [8]

Integration and rearrangement of equation [8] yielded
the following equation

t
/ qt = 1 / k2qe2 + 1 / qe t [9]

Where,
k2 is equilibrium rate constant of second order
kinetics model (g mg-1 min-1), qe is
the equilibrium capacity and qt is the biosorption capacity
at any time t. The time, t was plotted against t/qt and
straight lines as shown in Figure 13 were
fitted to these plots. The correlation coefficients of 0.99 indicated
the applicability of pseudo second order model to the present system.
The applicability of this model suggested that biosorption of lead(II)
and chromium(VI), on groundnut hull was based on chemical reaction,
between metals and active sites of the biosorbent. The elemental
analysis and FTIR spectra of groundnut hull also supported this
argument. The equilibrium rate constant, k2 and equilibrium
capacity, qe were determined
from the slope and intercept of the lines in Figure
13. The pseudo second
order model parameters are listed in Table 2.
The values of the equilibrium capacity evaluated from the pseudo
second order model were in close agreement to those determined experimentally.
The comparison of experimental data and the pseudo second order model
is shown in Figure 5. This
comparison revealed that model gave a good fit of the experimental
data points.

Effect of temperature

The effect of temperature on biosorption of
lead(II) and chromium(VI), was studied by varying the temperature in the range
20-45ºC. As depicted by Figure 14, the change in temperature effected
the biosorption of lead(II) and chromium(VI) differently. The biosorption of lead(II)
decreased with increasing temperature from 20-45ºC, while that of chromium(VI)
increased with increasing temperature up to 40ºC and then started decreasing.
The temperature higher than 40ºC caused a change, in the texture of the biomass
and thus reduced its biosorption capacity. The reason for having different behaviour
of lead(II) and chromium(VI) biosorption with temperature may be due to the
different mechanisms involved in the biosorption of these metals. The
biosorbent contains more than one type of sites for metal binding. The effect
of temperature on each site is different and contributes to overall metal
uptake.

The effect of temperature on biosorption
depends on the enthalpy change. The biosorption equilibrium constant, kd is described thermodynamically by Van’t Hoff equation as (Verma et al. 2006; Senthilkumar et al. 2007).

ln
kd = -∆H /
RT + ∆S / R [10]

The values of Gibbs free energy can be
calculated by using the following equation.

∆G = -RT ln kd [11]

Where, ∆H, ∆S and ∆G are
the change in enthalpy, entropy and Gibbs free energy of the system,
respectively. T is the absolute temperature (K), R is the gas constant (8.314 J
mole-1 K-1) and kd is the equilibrium constant
given by the following equation (Bektas et al. 2004).

kd = qe /
Ce[12]

Values of kd (lg-1)
were calculated at different temperatures using the Equation 12. The plot
of reciprocal temperature (1000/T) versus ln kd yielded straight lines,
as shown in Figure 15, with correlation coefficient 0.96 for lead(II)
and 0.95 for chromium(VI). The values of ∆H and ∆S were determined
from the slope and intercept of lines in Figure 15. The values of
∆G at different temperatures were calculated using Equation 11.
The values of ∆H, ∆S and ∆G are listed in Table 2.

Positive values of ∆H, ∆G and higher
removal capacities at elevated temperatures, indicated that biosorption of chromium(VI)
was endothermic in nature. At higher temperatures the energy of system seemed
to facilitate the chromium(VI) attachments onto biosorbent surfaces. On the
other hand enthalpy change for lead(II) was negative and a decrease in removal
capacity was observed with increase in temperature from 20ºC to 45ºC. This
showed that biosorption of lead(II) was exothermic.

Groundnut
hull was evaluated as possible biosorbent for removal of lead(II)
and chromium(VI) from wastewater. The maximum biosorption capacity
of lead(II) and chromium(VI) was 31.54 ± 0.63 and 30.21 ± 0.74
mg g-1, respectively. The biosorption was dependent
on pH and optimal pH for lead(II) and chromium(VI) was 5 and 2, respectively.
The equilibrium and kinetics data at different temperatures was
used to evaluate the thermodynamics parameters. The thermodynamics
studies revealed that biosorption of chromium(VI) was endothermic,
while biosorption of lead(II) was exothermic. The biosorption followed
the Langmuir model. The kinetics of biosorption was well represented
by Pseudo second order kinetic model. The release of Ca, Mg and
Na and K ions in lead(II) biosorption and OH- ions in
chromium(VI) biosorption, revealed that ion exchange was the major
removal mechanism.

Acknowledgements

The
authors are thankful to Mr. Umar, Mr. Zafar and Mr. Furqan for
providing the analytical facilities. Special thanks are for Lt.
General (R) Muhammad Akram Khan, the worthy Vice Chancellor, University
of Engineering and Technology Lahore, for his encouragements and
providing the facilities for this research.